Any data processing terminal device, ranging from a simple calculator to a complex point-of-salt terminal, can be viewed as a combination of functional units (controllers, printers, displays, etc.), each of which has a dedicated function to perform as well as a need to exchange data and control signals with at least some of the other functional units.
Conventionally, the functional units have been linked through electrical wires usually in the form of electrical cables. Cables have known disadvantages. Cables usually represent a significant portion of the component cost of a terminal device. The time and labor required to assemble terminal devices with electrical cables are also normally significant. Moreover, electrical cables are susceptible to electric noise or stray signals which can produce errors in the data or control information being transmitted between functional units.
To avoid some of these problems, proposals have been made that optical fibers be used to provide communications links between the functional units. Optical fibers are less susceptible to noise or stray signals than electrical cables but cost more and are harder to work with during terminal assembly operations.
To overcome some of the known problems of electrical or optical cables, a cable-free chamber has been disclosed as an optical communications link between functional units in a terminal device.
U.S. Pat. No. 4,063,083 discloses a system having an optical link in which a single beam of optical energy is transmitted along a straight line within a closed chamber from one pluggable card to the next. At each card the optical energy may be protected, modified and retransmitted to the next card along the line. Lenses are included for maintaining the beam focus.
While the approach disclosed in this patent avoids some of the problems inherent in the use of electrical or optical cables, certain other problems seem to be created. The terminal device would have to be carefully assembled to maintain the proper beam alignment. Also, the arrangement is somewhat inflexible in that the cards must be arranged in series along the beam path.
An alternative approach to closed chamber optical communications is disclosed in co-pending application Ser. No. 362,681, filed Mar. 29, 1982, and assigned to the assignee of the invention. According to that application, one or more functional units can be plugged into a frame adjacent a substantially enclosed optical chamber. A transducer on each functional unit can inject optical energy into the chamber through an opening in one wall thereof and can detect optical energy injected by other transducers through similar openings. Within the chambers, injected optical signals are diffusely reflected from the interior walls and are "scattered" to other transducers.
Co-pending application Ser. No. 362,681, filed Mar. 29, 1982, now U.S. Pat. No. 4,527,285, discloses a different terminal structure in which optical energy injected into the chamber is not diffusely reflected but is instead confined to predetermined paths by ellipsoid parabolic reflectors, preferably molded integrally with the floor of the optical chamber.
Prior art terminal devices using optical communications have not addressed the problem of interfacing remote functional units to the remainder of the terminal. Remote functional units are units which are logically an integral part of the terminal structure but are physically located at some distance from the remainder of the terminal. Remote functional units are connected to the terminal by means of optical or electrical cables. In a supermarket environment, produce scales and bar code scanners are examples of remote functional units. In a retail store environment, a display might be located remotely from the remainder of the terminal.
The straightforward approach to interfacing remote functional units to the remainder of a closed chamber communications terminal would be to provide complete, self-contained interface circuits for all of the remote functional units. That is, each interface circuit would include both an optical emitter and an optical detector in communication with the optical chamber.
There are drawbacks to this straightforward approach. The cost of having complete, self-contained interface circuits for all remote units would be high. Moreover, it would very likely be difficult to physically locate all of the necessary interface circuits in the optical chamber in locations in which optical signals could be received at suitable levels and in which one unit would not "shade" or partially block signal paths to one or more other remote functional units.